Saturday, July 27, 2013


When N631S and I departed KVKX last Monday for the trip north to Connecticut, Bridgeport (KBDR) was reporting a 900 foot overcast that was forecast to improve to 1,500 broken around 9AM local time. There were no NOTAM's related to the ILS, so it looked like a good situation.

By the time we were abeam Atlantic City the overcast at KBDR had dropped to 700 feet and by the time we were over the top of JFK it was down to 300 – which is the Decision Altitude for the ILS Runway 6 approach. But there we were, so we flew the approach to have a look. At 300 feet there was nothing but gray in the windshield and we proceeded to fly the missed approach. Checking back on with New York, we heard the expected, "Say intentions." My reply was "Skylane 31 Sierra would like to divert to Oxford." Oxford was reporting a 900 foot overcast.

The controller said, "Maintain 3,000 feet, fly heading 250, vectors for the ILS 36 approach at Oxford." Which will bring us to the point of the story.

New York vectored us onto the final approach course for the ILS Runway 36 approach and handed us off to Oxford Tower. I checked in outside the Final Approach Fix (FAF) and was cleared to land. As usual, I was flying the ILS with 10° of flaps and about 14 inches manifold pressure. This nicely results in about 95 knots and a 600 to 700 feet per minute descent.

We broke out of the schmoo at about 800 feet; there was the runway. At about 500 feet I pitched the nose up a bit to slow N631S and reached over to move the flap control to the fully down position. And...nothing happened. To my surprise, with the flap control fully down, there was no additional flap deployment at all. The flaps just sat there at about 10° extension.

Oxford (KOXC) has about 7,000 feet of runway and landing with 10° flaps is a non-event. In fact, the landing was excellent...a "greaser" (more on that in a bit). I taxied over to the FBO and tried cycling the flaps. Nothing I did would produce more than 10° of movement. So I waited for the ceiling to lift in Bridgeport (which took about an hour), and flew back down there – executing another non-full-flaps landing on arrival. Which led to the next surprise.

The landing at KBDR was not as pretty as the one in Oxford had been; in fact it was a bit of an 'arrival'. With 10° of flaps deployed and the flap control lever in the fully lowered position, the main gear contacted the runway with a hefty 'thump'...and the rest of the flaps deployed very nicely as I rolled down the runway.

Taxiing to the tiedown, I cycled the flaps up and down and got nothing but 'ops normal'. Lever up, flaps up; lever fully down, flaps fully down. All working the way it's supposed to. Drat! I secured N631S and found Tony the Mechanic in the maintenance hangar. He listened to my story and agreed that there was nothing to be done until the failure showed up again.

So...on Wednesday morning, I went back to the airport, opened up N631S, turned the Master Switch ON, and moved the flap control all the way down. The motor began to run, the flaps deployed to 10° – and stopped! Yay! I promptly secured the airplane and went to find Tony.

"Hey, Tony," I called, "Good news – the flaps aren't working!" This drew a strange look from a couple of other people, and a smile from Tony who said, "Good...we'll pull it in here gently, and look for the problem." I guess that it's only in the world of maintenance that the recurrence of a problem is a good thing and its absence is a bad thing. Pilots and mechanics know perfectly well that the problem WILL return...probably when you really don't need it.

The next morning, Tony reported that the problem was fixed. "I turned the master on, and reached up under the panel and moved some wires and the motor started and lowered the flaps. Aha! Pulled the seat out and got under there. A wire to one of the switches was almost broken but not quite – that's why it was intermittent."

Have a look at the picture. When you lower the flap control, it moves the cam and that closes the 'flap down' switch (green in the picture). As the flaps go down, the follower mechanism rotates the switch mount arm and the position indicator until the switch opens and the flaps stop moving – at the ordered position. (It was a wire to this switch that had failed.) When you're done with the flaps, the cam moves the other way, closes the 'flap up' switch, and the process runs in the opposite direction. They've got clever designers out in Wichita!

So N631S is completely healthy again, I get to land with all the flaps I want, and the fix was easy and fairly painless. One of the joys of curating a 36 year old aircraft, I suppose.


Cedarglen said...

I enjoy your posts, Frank. There is always a careful description of the issue and a thoughtful analysis of severity and options. Mechanic "Tony" and his colleagues must enjoy your attention to detail. That's far more helpful that tossing the keys and commenting,"the flaps don't work right." The only part that I do not understand
is the extra leg from KOXC to KBDR with an obvious flap fault. This is certainly not a challenge or a debate, Frank; I simply do not understand this piece of your otherwise faultless logic. Can you help me out a bit? Thanks for another good one, sir. When you write, I learn. Best wishes, -C.

Frank Van Haste said...

Happy to oblige, Craig. At OXC, I had repeatedly cycled the flaps between full up and 10 degrees. Very consistent. Heading back to Bridgeport, expecting the ILS 6, I knew I had 4,700+ feet of runway. If the flaps packed it in completely, a no-flaps landing would be routine. If the guy in front of me pranged and closed the airport, my alternate was right back to 7,000 feet of runway at OXC. Or Bradley if need be. Plenty of outs.

The only flap fault that would be a definite AOG (airplane-on-ground) problem, in my view, would be any asymmetry. At no point did I see any hint of that sort of issue.

So, to me, the leg back to BDR was nowhere near the edge of the envelope.

Best regards,


Cedarglen said...

Thanks. Understood. -C.